Owing to its superior mechanical properties, graphene has been used to reinforce and substantially improve the strength of composite materials. Still lacking, however, is a clear understanding of graphene's reinforcing mechanism at the atomic level, especially in relation to its pull-out behavior. By molecular dynamics (MD), it is found that pull-out of graphene, different from that of micro fibers, is not governed by friction only. Rather, the pull-out force is revealed to be governed by a “crack surface adhesion” phenomenon due to unbalanced adhesion at the crack surface when graphene is not functionalized and the crack opening rate is small. Crack surface adhesion produces a constant pull-out force (about 0.2–1 N per meter width) regardless of the embedded length. There is a transition from crack surface adhesion governed pull-out to friction governed pull-out when the crack opening speed, graphene size and degree of functionalization increase. A new model is developed to integrate friction and crack surface adhesion with the 2D geometry of graphene. The new model can be used to predict the crack bridging stress for 2D graphene (or other 2D atomic thin reinforcements). The outcome of this study benefits the understanding and design of new graphene composites.